The present invention relates to a method and apparatus for locating a cased well from another well, and particularly for detecting and locating a blowing well from a relief well.
When a blowout occurs in a well, the conventional technique consists in shutting the well by using a relief well drilled from a point which is at a distance from the blowout zone. This technique requires the relief well to be drilled in the direction of the blowing well, which assumes that this direction can be determined from within the relief well.
Various methods have been proposed for locating a blowing well. A description of these techniques can be found in U.S. Pat. No. 4,372,398, column 1, line 55 to column 2, line 65 which description is incorporated herein by reference. The type of technique to which the invention pertains makes use of the presence of a metal casing in the blowing well, which casing therefore conducts electricity.
U.S. Pat. No. 4,072,200 describes a locating method consisting in exciting the casing of the target well with an alternating current so as to set up a magnetic field around the casing, and in detecting this field by means of a sensor placed in another well. That patent specifies that such a method is not applicable if the target well is on fire since it is then not possible to excite the casing of the target well.
The above-mentioned U.S. Pat. No. 4,372,398 describes a method of this type in which the electrical excitation is achieved by means of an electrode lowered down the relief well. The magnetic field set up around the casing in the target well is detected by means of sensors placed in the relief well at a distance from the electrode which is large enough to ensure that the currents flowing in and around the relief well do not disturb detection. However, this method suffers from short range limitations: it is hardly possible to detect a target well which is more than a few tens of meters away.